Such an apparatus including an illumination unit and intended to manufacture integrated semiconductor circuits is known from U.S. Pat. No. 5,166,530. In this apparatus a mask is illuminated and imaged in a repetitive manner on a photoresist layer provided on a semiconductor substrate. A large number of ICs must be formed on the substrate. After an image of the mask has been formed on the substrate, this substrate is to this end displaced with respect to the mask by a distance which is slightly larger than the length or the width of the ICs to be formed, whereafter a subsequent mask image is made, and so forth. It is desirable that the illumination beam then has an intensity which is high as possible so that the illumination time for each IC is as short as possible and the time of passage of the substrate through the apparatus, i.e. the time required to illuminate all ICs is as short as possible. Consequently, a radiation source having a high radiation power must be used. Such a radiation source must be cooled because the greater part of the energy supplied to the radiation source is converted into heat. Moreover, the optical system within the illumination housing should be and should remain efficient for concentrating and further guiding the source radiation, i.e. it should collect as much radiation from the source as possible and absorb or deflect as little usable radiation as possible. For this reason also other optical components in the illumination housing, arranged in the path of the illumination beam, should preferably be cooled so as to prevent their degradation.
It has already been proposed in U.S. Pat. No. 5,166,530 to cool the radiation source with air from the ambience of the illumination housing, which air is passed through this housing for this purpose. However, it has been found that the power of the illumination beam then decreases relatively fast with time. In accordance with U.S. Pat. No. 5,166,530 this decrease is caused by the fact that the ambient air comprises volatile or gaseous particles such as hexamethyl disilazane (HMDS) coming from the adhesive layer on the substrate which ensures the adhesion of the photoresist layer, from which particles via photopolymerization silicon oxide is produced which precipitates on, for example the radiation-collecting reflector arranged behind the radiation source. According to U.S. Pat. No. 5,166,530, this SiO.sub.2 layer has a high absorption coefficient for radiation at a wavelength of less than 365 nm. Since an illumination beam having an increasingly shorter wavelength is to be used for projecting images having increasingly smaller details on the substrate, the precipitation of SiO.sub.2 within the illumination housing and on surface areas present in the path of the illumination beam is a serious problem.
To solve this problem, a carbon filter absorbing the volatile particles is arranged in the illumination unit disclosed in U.S. Pat. No. 5,166,530 in front of the inlet opening of the illumination housing. However, such a filter will be saturated in the course of time, which means that the particles are passed after some time in ever increasing quantities so that the power of the illumination beam decreases and the filter must eventually be replaced. A fresh carbon filter of good quality, used in optimal circumstances cannot purify the ambient air from the particles for 100%. In order that the period of time the air stays within this filter is sufficiently long, the carbon filter should be sufficiently thick so that this filter requires extra space. For a strong cooling of the radiation source a large quantity of air must be blown through the illumination housing, for example by means of a fan. However, the air then stays within this filter for a short period of time and the air is less well purified, for example for 60%. To achieve that the illumination housing contains only air which has passed through the filter, this housing should be airtight, which involves an extra complication for the housing.
It has been found that in the decrease of the power of the illumination beam not only SiO.sub.2 but aim other compounds of silicon and oxygen such as Si.sub.3 O.sub.4 and generally Si.sub.x O.sub.y may play a role and that these compounds form a diffuse layer on, inter alia the radiation source reflector. Moreover, such a layer disturbs the specific reflector layer structure required for the reflection, so that the reflection wavelength band will be narrower. It has also been found that the HMDS particles may also decompose under the influence of radiation having a wavelength in the deep infrared range.